Processes of preparing cellulosic pulps including kraft pulping process are within the skill in the art for instance as discussed in Casey, Pulp and Paper; Chemistry and Chemical Technology, 3rd ed., vol. 1, (1980) especially pages 291-491 and 504-567. In chemical pulps, the wood or other cellulose source is advantageously separated into pulp with the help of chemicals. Two principal chemical pulping methods are in use: soda process and kraft process. The soda process utilizes a strongly alkaline solution of sodium hydroxide to digest wood chips. Kraft pulping process utilizes sulfate materials (reduced to sulfites in the furnace) and hydroxides. The kraft pulping process is widely used these days for producing pulp for subsequent processing to produce paper for instance as discussed in Gary A. Smook, "Handbook for Pulp and Paper Technologists", Second Edition, Angus Wilde Publications, pp. 74-83 and 133-162 (1992).
In this process, wood chips are cooked (digested) under conditions of heat and pressure using "white liquor" containing sodium hydroxide (NaOH) and sodium sulfide (Na.sub.2 S) to release cellulose fibers from other components such as lignin. After digestion, fibers are commonly released under pressure into a tank in a process referred to as blowing or blowdown. Then the pulp is washed to remove spent chemicals, lignin and other organic chemicals. The liquid removed from the pulp is referred to as "black liquor" and contains about 25 percent dissolved solids. The black liquor resulting from the washing stage is concentrated by evaporation to a desired concentration and burned to reclaim the inorganic chemicals and provide fuel value. The organic materials are advantageously incinerated to yield an inorganic smelt of sodium carbonate and sodium sulfide. The resulting inorganic smelt is dissolved to form "green liquor". Clarified green liquor is reacted with lime (CaO) in a slaker (causticizer). This produces the white liquor (which is reused in the digestion step) and lime mud. The lime mud is then recalcined in a heated lime kiln to recover lime (CaO) which is used in a slaker.
An important step in the processing of lime mud is lime mud dewatering. This is typically done using a suitable filtration means such as vacuum drum filter. Typically, a rotary vacuum drum filter is used for dewatering lime mud and washing it just prior to its entrance into the lime kiln. Lime mud from storage is diluted to 25-35 percent solids and pumped to the "precoat" filter. This filter operates at 15-20 inches of vacuum (about 9 psia). The drum is covered with a screen made of stainless steel or plastic fiber (typically 150 mesh). A cake of lime mud builds up on the screen as the drum turns, and a doctor blade is fixed at a distance of 3/8-5/8 inch (about 0.94-1.56 cm.) from the screen. Consequently, a layer of lime mud remains on the screen continually and acts as the filter medium for the lime mud. Thus the name "precoat" filter. This "precoat" enhances the filter's ability to remove fine particles during the filtration process. During the filtration, as the lime mud builds up, the doctor blade removes it and the dewatered lime mud falls onto a screw feeder which transports it to the feed end of the kiln. Dewatered lime mud is typically about 65-75 percent solids. The temperature at the pre-coat filter is important. Best results are seen with temperatures of about 70.degree. C., while cold temperatures can reduce filter capacity by 10 percent or more.
However, many filters are not efficient and the use of dewatering additives would be desirable to facilitate removal of water from and improve filtration of the lime. The use of dewatering additives in the lime mud processing would not only improve filtration of lime mud but would also result in less energy required for heating the lime kiln to convert lime mud to lime.
The main benefit of using lime mud dewatering additives would be the reduction of the water content of lime mud exiting the filter. Additional benefits may be any of the following: reduction of fuel consumption in the lime kiln resulting in the energy savings; reduction of formation of "rings" and "balls" in the kiln (better water removal results in greater removal of the water-soluble salts responsible for these formations in the kiln); reduction of sulfur stack emissions from the process (much of sulfur is present at this point in the process as sodium salts; better dewatering results in more efficient sulfur removal); increased lime mud filter runability (a drier filter cake results in less plugging of the screw feed that transports dewatered lime mud to the kiln resulting in lower maintenance and less needed cleanup).
Thus, there is a clear need in the cellulosic pulp industry for an additive which will enhance dewatering of lime mud.